how can I make collision work for this beam particle with the brushes around or the AI character?
You can’t make the beam collide with anything. You need to make a linetrace then set its hit location as the beam target.
Can you please tell me how can I get access to the beam target?
Firstly, when you make the beam, there was a source and target right?
Secondly, when you make the spawn emitter for the beam in your spawner, gun or player, you can search for the node called set beam source point and set beam target point.